Speeding SQL queries for a large database?

Your log shows that MySQL is performing a linear scan of all table records to get the necessary row – this is because there is no index for guid

Assuming wp_rb_posts is just the normal posts table with a slightly modified prefix, then you can just add an index for that field, and you’ll see a dramatic improvement.

The SQL statement for doing this is:

CREATE INDEX guid ON wp_rb_posts(guid);